While looking through the undead troops i noticed that there is no true mana generator for them. There are troops that turn colour x to colour y to either increase damage or as a side effect of the cast, and keeper of souls can turn any colour to skulls, however, these effects are more aimed around damage dealing than mana generation. As such I thought of the viod orb, a construct, designed to absorb power and turn it into fuel for various undead armies. Over time, as the orb consumed life, it drew the attention of lost undead souls who have been caught inside it and given the inanimate orb a malicious intent and desire to absorb all life.

Stats:

attack = 0 (it’s inanimate)
def, life and magic should be similar to other undead lenegdaries, with possibly more focus on life

traits

necromany
magic bond (1 extra puple mana)
undying

spell: 15 (standard for legendaries) blue and brown mana

first effect
either: explode all purple gems OR convert target mana colour to purple
second effect
deal [magic/4 + souls/10] damage to all enemies and [give life/heal] undead for the same amount